Subtropical stratus clouds—low-lying, relatively thin clouds often referred to as “aboveground fog”—regulate coastal, and even global, climate. Small decreases in the occurrence of these near-surface clouds would drive up temperatures and exacerbate drought conditions, but climate models are not consistent in predicting the impact of stratus clouds. Looking back at historical records to analyze past cloud cover and temperature trends could give scientists important insights into how the clouds may be influencing climate conditions.

Williams et al. pored through 67 years’ worth of hourly observations of fog, stratus cloud frequency, and stratus cloud base height taken from 24 airfields across coastal southern California, clustered in four areas: Los Angeles, San Diego, Santa Barbara, and two islands west of Los Angeles. The researchers found that between 1948 and 2014, stratus frequency decreased by 23% in the Los Angeles area. Declines were nearly as common in San Diego but did not occur in Santa Barbara or the islands.

Even larger trends were observed with fog. Los Angeles saw a 63% reduction in fog frequency, whereas San Diego experienced less severe fog reductions, and fog frequency remained stable in Santa Barbara and increased over the islands. These trends were most pronounced in the early morning, when stratus clouds and fog are most common.

More urbanized areas had larger increases in cloud base height and larger decreases in the frequency of fog and low stratus clouds.The trends match changes observed in cloud base heights. Base heights have risen in Los Angeles by an average of 12.7 meters per decade. These heights increased in San Diego to a lesser extent but remained the same over Santa Barbara, whereas they have actually fallen over the islands.

The historical cloud records are consistent with an urban heat island effect: Increases in nighttime and early morning warming prevent water vapor from condensing out of the air into clouds near the surface of the Earth. Higher temperatures essentially lift the altitude of condensation and cloud base height, thereby reducing fog frequency. The researchers found that, in general, the more urbanized areas in the study had larger increases in cloud base height and larger decreases in the frequency of fog and low stratus clouds.

This pattern is troubling because fewer daytime clouds, and resultant decreases in shading and water deposition to ecosystems, can cause warming and drought. Warming further reduces fog and stratus cloud formation. This positive feedback loop can lead to increases in water and energy demand, wildfire risk, and adverse public health consequences. However, from the perspective of transportation safety and tourism, these changes in cloud base height could have positive effects. (Geophysical Research Letters, doi:10.1002/2014GL063266, 2015)

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